You can always change the default value of the brightness setting. The brightness is set to a default to suit all users. You will always have the ability to change the brightness.

Hi, my point is default value is too high, it affect battery life. user some time forget set this value, so i think this value should be changed by default. now, i set this value to 0% indoor, i can see my flame screen clearly with 0%

I don't believe the default will be changed. It's intended and designed for users to adjust based on their needs.

Hi sailfish009 and Andrew, Thank you for your question, please file a bug for the default brightness display at bugzilla.mozilla.com.